Function programming in Java 17
- Tram Ho
Functional interfaces:
| Interface | Method | Return type |
|---|---|---|
Supplier<T> | get() | T |
Consumer<T> | accept(T) | void |
<BiConsumer<T, U> | accept(T, U) | T |
Predicate<T> | test(T) | boolean |
BiPreidcate<T, U> | test(T, U) | boolean |
Function<T, R> | apply(T) | R |
BiFunction<T, U, R> | apply(T, U) | R |
UnaryOperator<T> | apply(T) | T |
BinaryOperator<T> | apply(T, T) | T |
For setting development for Java 17, see https://stackoverflow.com/questions/70083274/java-17-java-invalid-source-release-7-with-enable-preview-preview-language/70083285#70083285
Use Supplier
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 | <span class="token keyword">package</span> <span class="token namespace">org<span class="token punctuation">.</span>example</span><span class="token punctuation">;</span> <span class="token keyword">import</span> <span class="token namespace">java<span class="token punctuation">.</span>time<span class="token punctuation">.</span></span><span class="token class-name">LocalDate</span><span class="token punctuation">;</span> <span class="token keyword">import</span> <span class="token namespace">java<span class="token punctuation">.</span>util<span class="token punctuation">.</span>function<span class="token punctuation">.</span></span><span class="token class-name">Supplier</span><span class="token punctuation">;</span> <span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">App</span> <span class="token punctuation">{</span> <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token class-name">Supplier</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">LocalDate</span><span class="token punctuation">></span></span> localDateSupplier <span class="token operator">=</span> <span class="token class-name">LocalDate</span><span class="token operator">::</span><span class="token function">now</span><span class="token punctuation">;</span> <span class="token class-name">Supplier</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">LocalDate</span><span class="token punctuation">></span></span> localDateSupplier1 <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-></span> <span class="token class-name">LocalDate</span><span class="token punctuation">.</span><span class="token function">now</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token class-name">LocalDate</span> localDate <span class="token operator">=</span> localDateSupplier<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token class-name">LocalDate</span> localDate1 <span class="token operator">=</span> localDateSupplier1<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">print</span><span class="token punctuation">(</span><span class="token string">"localDate = "</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>localDate<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">print</span><span class="token punctuation">(</span><span class="token string">"localDate1 = "</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>localDate1<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token punctuation">}</span> |
result
1 2 3 | localDate <span class="token operator">=</span> <span class="token number">2021</span><span class="token operator">-</span><span class="token number">11</span><span class="token operator">-</span><span class="token number">23</span> localDate1 <span class="token operator">=</span> <span class="token number">2021</span><span class="token operator">-</span><span class="token number">11</span><span class="token operator">-</span><span class="token number">23</span> |
More complete object
1 2 3 4 5 6 7 8 9 10 11 12 | <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> stringArrayList <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation"><</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> stringArrayList<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">"Do Nhu Vy"</span><span class="token punctuation">)</span><span class="token punctuation">;</span> stringArrayList<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">"Nguyen Bich Van"</span><span class="token punctuation">)</span><span class="token punctuation">;</span> stringArrayList<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">"Nguyen Hong Do"</span><span class="token punctuation">)</span><span class="token punctuation">;</span> stringArrayList<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">"Nguyen Tien Nam"</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token class-name">Supplier</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">ArrayList</span><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span><span class="token punctuation">></span></span> arrayListSupplier2 <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-></span> <span class="token punctuation">{</span> <span class="token keyword">return</span> stringArrayList<span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token punctuation">;</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> strings2 <span class="token operator">=</span> arrayListSupplier2<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">print</span><span class="token punctuation">(</span><span class="token string">"strings2 = "</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>strings2<span class="token punctuation">)</span><span class="token punctuation">;</span> |
or other way
1 2 3 4 5 6 7 8 9 10 11 12 | <span class="token class-name">Supplier</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">ArrayList</span><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span><span class="token punctuation">></span></span> arrayListSupplier2 <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-></span> <span class="token punctuation">{</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> stringArrayList <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation"><</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> stringArrayList<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">"Do Nhu Vy"</span><span class="token punctuation">)</span><span class="token punctuation">;</span> stringArrayList<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">"Nguyen Bich Van"</span><span class="token punctuation">)</span><span class="token punctuation">;</span> stringArrayList<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">"Nguyen Hong Do"</span><span class="token punctuation">)</span><span class="token punctuation">;</span> stringArrayList<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">"Nguyen Tien Nam"</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">return</span> stringArrayList<span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token punctuation">;</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> strings2 <span class="token operator">=</span> arrayListSupplier2<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">print</span><span class="token punctuation">(</span><span class="token string">"strings2 = "</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>strings2<span class="token punctuation">)</span><span class="token punctuation">;</span> |
result
1 2 3 | strings <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span> strings2 <span class="token operator">=</span> <span class="token punctuation">[</span>Do Nhu Vy<span class="token punctuation">,</span> Nguyen Bich Van<span class="token punctuation">,</span> Nguyen Hong Do<span class="token punctuation">,</span> Nguyen Tien Nam<span class="token punctuation">]</span> |
How Java virtual machine manage object what is instance of Supplier interface? Try an experiment:
1 2 3 | <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">print</span><span class="token punctuation">(</span><span class="token string">"arrayListSupplier2 = "</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>arrayListSupplier2<span class="token punctuation">)</span><span class="token punctuation">;</span> |
result
1 2 | arrayListSupplier2 <span class="token operator">=</span> org.example.App<span class="token variable">$$</span>Lambda<span class="token variable">$19</span>/0x0000000800c03218@448139f0 |
Implicity way, this is the result of arrayListSupplier2.toString() . Symbol $$ means object exist on memory only, not in hard disk. Exactly, arrayListSupplier2 initialized by a lambda expression as you seen in the below.
Implement Consumer interface
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 | <span class="token keyword">package</span> <span class="token namespace">org<span class="token punctuation">.</span>example</span><span class="token punctuation">;</span> <span class="token keyword">import</span> <span class="token namespace">java<span class="token punctuation">.</span>util<span class="token punctuation">.</span></span><span class="token class-name">HashMap</span><span class="token punctuation">;</span> <span class="token keyword">import</span> <span class="token namespace">java<span class="token punctuation">.</span>util<span class="token punctuation">.</span>function<span class="token punctuation">.</span></span><span class="token class-name">BiConsumer</span><span class="token punctuation">;</span> <span class="token keyword">import</span> <span class="token namespace">java<span class="token punctuation">.</span>util<span class="token punctuation">.</span>function<span class="token punctuation">.</span></span><span class="token class-name">Consumer</span><span class="token punctuation">;</span> <span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">SampleConsumer</span> <span class="token punctuation">{</span> <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token class-name">Consumer</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> stringConsumer <span class="token operator">=</span> <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token operator">::</span><span class="token function">println</span><span class="token punctuation">;</span> <span class="token class-name">Consumer</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> stringConsumer1 <span class="token operator">=</span> vy <span class="token operator">-></span> <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>vy<span class="token punctuation">)</span><span class="token punctuation">;</span> stringConsumer<span class="token punctuation">.</span><span class="token function">accept</span><span class="token punctuation">(</span><span class="token string">"Nguyen Bich Van"</span><span class="token punctuation">)</span><span class="token punctuation">;</span> stringConsumer1<span class="token punctuation">.</span><span class="token function">accept</span><span class="token punctuation">(</span><span class="token string">"Tran Phuong Ly"</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">var</span> map <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">HashMap</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">,</span> <span class="token class-name">Integer</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token class-name">BiConsumer</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">,</span> <span class="token class-name">Integer</span><span class="token punctuation">></span></span> stringIntegerBiConsumer <span class="token operator">=</span> map<span class="token operator">::</span><span class="token function">put</span><span class="token punctuation">;</span> <span class="token class-name">BiConsumer</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">,</span> <span class="token class-name">Integer</span><span class="token punctuation">></span></span> stringIntegerBiConsumer1 <span class="token operator">=</span> <span class="token punctuation">(</span>ke<span class="token punctuation">,</span> va<span class="token punctuation">)</span> <span class="token operator">-></span> map<span class="token punctuation">.</span><span class="token function">put</span><span class="token punctuation">(</span>ke<span class="token punctuation">,</span> va<span class="token punctuation">)</span><span class="token punctuation">;</span> stringIntegerBiConsumer<span class="token punctuation">.</span><span class="token function">accept</span><span class="token punctuation">(</span><span class="token string">"pigeon"</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span> stringIntegerBiConsumer1<span class="token punctuation">.</span><span class="token function">accept</span><span class="token punctuation">(</span><span class="token string">"pig"</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">print</span><span class="token punctuation">(</span><span class="token string">"map = "</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>map<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token punctuation">}</span> |
Result
1 2 3 | Nguyen Bich Van <span class="token keyword">Tran</span> Phuong Ly map <span class="token operator">=</span> {pigeon<span class="token operator">=</span><span class="token number">2</span><span class="token punctuation">,</span> pig<span class="token operator">=</span><span class="token number">4</span>} |